This invention relates to fasteners for securing covering materials to cementitious substrates. More particularly, this invention relates to an improved fastener of the sheet metal type, designed particularly for securing sheet coverings such as "base plys" to cementitious substrates such as lightweight insulating concretes while the cementitious substrate is in a soft, notfully cured condition.
In U.S. Pat. No. 3,710,672 to Robert L. Hallock, a sheet metal nail is described which is particularly useful for securing protective sheet coverings such as "base plys" to lightweight insulating concretes. These concretes are mixtures principally of lightweight aggregate such as expanded vermiculite, Portland cement and water. Base ply coverings are applied to the cast concretes before such have fully hardened, and the sheet metal nail described in the aforementioned Hallock patent is particularly effective in attaching the covering at this stage, that is while the concrete is still soft or "plastic". The fastener easily penetrates the covering and the soft concrete mass, spreading as it penetrates to insure a positive gripping in the subsequently hardened mass. Because of its special design, no void is left in the penetrated surof the concrete mass by the fastener, thereby eliminating an initial "yield", as well as offering greater resistance to the withdrawal of the fastener.
As described in U.S. Pat. No. 3,710,672, the Hallock sheet metal fastener has a head which also serves as a washer for holding the sheet covering, and a pair of opposed, generally U-shaped legs connected at one end to the head and terminating in penetrating ends. Each of the legs has a pair of longitudinally straight, parallel sides and a curved intermediate wall portion connecting the parallel sides. The legs further are designed such that the straight sides of one are received substantially their full width within the straight sides of the other. The opposed intermediate curved portions of the legs are separated at their head end and at their penetrating end substantially the same distance apart, the curved intermediate portion of each leg being curved inwardly. The legs are hinged to the head portion, this being the only means of connection with the head portion and to one another. Thus the legs move freely upon insertion in the concrete mass, the direction of movement being outwardly, that is, away from one another due to the curvature of the intermediate portions of the legs. Other features and advantages of the Hallock fastener are spelled out more fully in the U.S. Pat. No. 3,710,672, the disclosure of which is hereby incorporated by reference.
The fastener described in the aforementioned Hallock patent has met with significant commercial acceptance. However, one difficulty encountered in handling and use of the fastener resides in the tendency of the legs to lose proper positioning relative to the head. Since the legs are attached to the head only by the hinged connections thereto and not connected to one another, nor in any other fashion restricted from spreading, the fastener head can easily become "tilted," that is be positioned at an angle other than 90.degree. to the direction of the center line of the legs. The results is that the head must be straightened in order to position and drive it properly using a manual hammer. Another undesired result occurs when automatic nailing machinery is used to insert the fastener. The disfigured fasteners cause jam-ups of the machinery, and sometimes cause the fastener to be crushed rather than driven because the legs do not enter the substrate "straight."